This invention relates generally to optical communication networks, and, particularly, to optical waveguide interference filters.
In present day optical communication networks, interference filters are extensively used for purposes of wavelength channel multiplexing, switching, and dispersion compensation. The most promising type of such filters is based on planar waveguide circuits, fabricated primarily of silica or silicon. One of their major drawbacks is their spectrum temperature dependence caused by the temperature dependence of the index of refraction in the material used to make the circuits. For this reason chips containing such circuits need to be temperature stabilized with a heater or a thermoelectric cooler (TEC). This requires electric power, control electronics for feedback, and management of the dissipated heat.
Therefore, it is advantageous to build temperature insensitive waveguide circuits. There are several approaches to making an athermal device, none of which are completely satisfactory. Some include the mechanical moving of the point where the input fiber contacts the waveguide circuit. This results in severe reliability problems.
Other approaches use a material with a different slope of the refractive index temperature dependence. An example of such a material is a polymer, such as silicone. Grooves are etched through the waveguides and are filled with the polymer. The downside of this method is that the grooves cause high insertion loss due to diffraction. Also, combining two materials in one chip decreases the die yield and reliability and makes the process more complicated and, thus, more expensive.
At the same time, a waveguide made entirely of polymers is being developed. In this case it may be possible to tailor the parameters of the polymer to achieve very small temperature dependence. However, up to now the polymer waveguide devices have failed to compare to silica waveguide devices in performance and reliability.
Thus, there is a need for ways to reduce the temperature sensitivity of optical waveguide interference filters.